Machine component and method for producing the same

ABSTRACT

A sprocket wheel, which is an example of the machine component, includes a base made of a first metal, and an overlay disposed in contact with the base to cover at least a part of a surface of the base. The overlay includes a matrix made of a second metal, and hard particles dispersed in the matrix. The surface of the overlay is a forged surface. The hard particles located in an overlay surface region within an average particle diameter of the hard particles from the surface of the overlay are arranged side by side while being embedded in the overlay.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of PCT Application No.PCT/JP2014/080540 filed Nov. 18, 2014.

TECHNICAL FIELD

The present invention relates to machine components and methods forproducing the machine components, and more particularly, to a machinecomponent that includes an overlay having hard particles dispersed in amatrix and a method for producing the machine component.

BACKGROUND ART

On a machine component, an overlay may be formed for improving wearresistance and other purposes. Overlays adoptable for improving the wearresistance include one having hard particles dispersed in a matrix madeof steel, for example. Such an overlay can be formed, for example, byoverlaying welding (see, for example, Japanese Patent ApplicationLaid-Open No. 2008-763 (Patent Literature 1) and Japanese PatentApplication Laid-Open No. H8-47774 (Patent Literature 2)).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No. 2008-763

Patent Literature 2: Japanese Patent Application Laid-Open No. H8-47774

SUMMARY OF INVENTION Technical Problem

In a machine component with an overlay having hard particles dispersedin a matrix, the hard particles may fall off from the overlay during theuse of the machine component. The machine component having such anoverlay may suffer deterioration in wear resistance when the hardparticles fall off from the overlay.

An object of the present invention is to improve wear resistance of amachine component by preventing the hard particles from falling off fromthe overlay.

Solution to Problem

A machine component according to the present invention includes: a basemade of a first metal; and an overlay disposed in contact with the baseto cover at least a part of a surface of the base. The overlay includesa matrix made of a second metal, and hard particles dispersed in thematrix. The overlay has a surface which is a forged surface. The hardparticles located in an overlay surface region are arranged side by sidewhile being embedded in the overlay, the overlay surface region being aregion within an average particle diameter of the hard particles fromthe surface of the overlay.

In the machine component of the present invention, the surface of theoverlay is a forged surface. The hard particles located in the overlaysurface region are arranged side by side while being embedded in theoverlay. This prevents the hard particles from protruding noticeablyfrom the surface of the overlay. As a result, the hard particles areprevented from falling off during the use of the machine component,leading to improved wear resistance of the machine component. Thus,according to the machine component of the present invention, it ispossible to improve wear resistance of the machine component. It shouldbe noted that the average particle diameter of the hard particles may beobtained by observing a cross section perpendicular to the surface ofthe overlay with an optical microscope, and by calculating an average ofthe diameters of ten hard particles observed.

In the machine component described above, the hard particles located inthe overlay surface region may be arranged in contact with the surfaceof the overlay. With this, the region of a hard particle exposed fromthe surface of the overlay becomes small, which prevents the hardparticle from falling off.

In the machine component described above, among the hard particleslocated in the overlay surface region, any hard particle having a regionexposed from the surface of the overlay may have an acute central angle(of less than 90°) corresponding to the region exposed from the surfaceof the overlay. With this, the region of a hard particle exposed fromthe surface of the overlay becomes small, which prevents the hardparticle from falling off.

In the machine component described above, in a region including aninterface between the overlay and the base, the overlay may include aprotrusion that protrudes toward the base. This prevents the overlayfrom coming off the base.

In the machine component described above, the protrusion may have atleast a part of the hard particle received therein. This more reliablyprevents the overlay from coming off the base.

The machine component described above may be used as a component of atracked undercarriage, as a bucket tooth, or as a tooth of a crusher.The above machine component, which is excellent in wear resistance, issuitable for these machine components requiring high wear resistance.

A method for producing a machine component according to the presentinvention includes the steps of: preparing a base member made of a firstmetal; forming an overlay to cover at least a part of a surface of thebase member; and forging the base member having the overlay formed, suchthat the overlay is worked. The step of forming the overlay includesforming the overlay including a matrix made of a second metal and hardparticles dispersed in the matrix.

In the machine component producing method of the present invention, thebase member having the overlay formed is forged such that the overlayhaving the hard particles dispersed in the matrix is worked. When theoverlay is worked by forging, the hard particles that were protrudingfrom the surface of the overlay are pressed into the matrix havingrelatively low hardness. Thus, in the vicinity of the surface of theoverlay, the hard particles are sufficiently embedded in the overlay. Asa result, the hard particles are prevented from falling off during theuse of the machine component, leading to improved wear resistance of themachine component. Thus, according to the machine component producingmethod of the present invention, it is possible to improve wearresistance of the machine component.

In the machine component producing method described above, the step offorging the base member having the overlay formed may include hotforging the base member having the overlay formed. Hot forging adoptedcan facilitate forging of the base member with the overlay formed.

In the machine component producing method described above, the machinecomponent may be used as a component of a tracked undercarriage, as abucket tooth, or as a tooth of a crusher. The above machine componentproducing method, capable of producing the above machine componentexcellent in wear resistance, is suitable for producing these machinecomponents requiring high wear resistance.

Effects of Invention

As is clear from the above description, according to the machinecomponent and its producing method of the present invention, it ispossible to improve the wear resistance of the machine component bypreventing the hard particles from falling off from the overlay.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the structure of a track traveldevice;

FIG. 2 is a schematic diagram illustrating operations of a sprocketwheel and bushings;

FIG. 3 is a schematic perspective view showing the structure of abushing;

FIG. 4 is a schematic cross-sectional view showing the contact state ofthe sprocket wheel and a bushing;

FIG. 5 is a schematic cross-sectional view taken along the line V-V inFIG. 4;

FIG. 6 is a schematic cross-sectional view showing the structure of anoverlay at and near its surface;

FIG. 7 is a schematic cross-sectional view showing the structure at andaround an interface between the overlay and a base;

FIG. 8 is a flowchart schematically illustrating a method for producinga machine component;

FIG. 9 is a schematic perspective view illustrating a method forproducing a sprocket wheel;

FIG. 10 is a schematic perspective view illustrating the method forproducing the sprocket wheel;

FIG. 11 is a schematic cross-sectional view illustrating a method forforming an overlay;

FIG. 12 is a schematic perspective view illustrating the method forproducing the sprocket wheel;

FIG. 13 is a schematic perspective view illustrating the method forproducing the sprocket wheel;

FIG. 14 is a schematic perspective view illustrating a method forproducing a bushing;

FIG. 15 is a schematic perspective view illustrating the method forproducing the bushing;

FIG. 16 is a schematic perspective view showing the structure of abucket of a hydraulic excavator;

FIG. 17 is a schematic plan view showing the structure of a tooth;

FIG. 18 is a schematic cross-sectional view taken along the lineXVIII-XVIII in FIG. 17;

FIG. 19 is a schematic cross-sectional view showing the structure of atooth of a Comparative example;

FIG. 20 is a schematic cross-sectional view illustrating a method forproducing a tooth;

FIG. 21 is a schematic cross-sectional view illustrating the method forproducing the tooth;

FIG. 22 is a schematic perspective view showing the structure of acrusher;

FIG. 23 is a schematic perspective view showing the structure of astationary-section external tooth of the crusher;

FIG. 24 is a schematic cross-sectional view taken along the lineXXIV-XXIV in FIG. 23;

FIG. 25 is a schematic cross-sectional view taken along the line XXV-XXVin FIG. 24;

FIG. 26 is a schematic perspective view illustrating a method forproducing a stationary-section external tooth of a crusher;

FIG. 27 is a schematic perspective view illustrating the method forproducing the stationary-section external tooth of the crusher;

FIG. 28 is a schematic cross-sectional view illustrating the method forproducing the stationary-section external tooth of the crusher;

FIG. 29 is a schematic cross-sectional view illustrating the method forproducing the stationary-section external tooth of the crusher;

FIG. 30 is a schematic perspective view illustrating the method forproducing the stationary-section external tooth of the crusher;

FIG. 31 is a schematic perspective view illustrating the method forproducing the stationary-section external tooth of the crusher;

FIG. 32 is a schematic cross-sectional view illustrating the method forproducing the stationary-section external tooth of the crusher;

FIG. 33 is a schematic cross-sectional view illustrating the method forproducing the stationary-section external tooth of the crusher;

FIG. 34 is a photograph showing a cross section of a tooth;

FIG. 35 is an optical micrograph showing a surface and its vicinity ofan overlay (Example);

FIG. 36 is an optical micrograph showing a surface and its vicinity ofan overlay (Comparative example);

FIG. 37 is an optical micrograph showing an interface between an overlayand a base and its vicinity (Example); and

FIG. 38 is an optical micrograph showing an interface between an overlayand a base and its vicinity (Comparative example).

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below. In thefollowing drawings, the same or corresponding parts are denoted by thesame reference numerals, and the description thereof will not berepeated.

First Embodiment

A machine component according to a first embodiment, which is anembodiment of the present invention, will be described taking a sprocketwheel and bushings of a track travel device as examples. FIG. 1 is aschematic diagram showing the structure of a track travel device. FIG. 2is a schematic diagram illustrating operations of a sprocket wheel andbushings. FIG. 3 is a schematic perspective view showing the structureof a bushing. FIGS. 4 and 5 are schematic cross-sectional views showingthe contact state of the sprocket wheel and a bushing.

Referring to FIG. 1, a track travel device 1 in the present embodimentis a travel device for a work machine such as a bulldozer, and includes:a track 2; a track frame 3; an idler tumbler 4; a sprocket wheel 5; aplurality of (here, seven) track rollers 10; and a plurality of (here,two) carrier rollers 11.

The track 2 includes a plurality of track links 9, which are connectedendlessly, and track shoes 6, which are fixed to the corresponding tracklinks 9. The track links 9 include outer links 7 and inner links 8. Theouter links 7 and the inner links 8 are connected alternately.

The idler tumbler 4, the plurality of track rollers 10, and theplurality of carrier rollers 11 are attached to the track frame 3 insuch a manner that they are rotatable about their respective axes. Thesprocket wheel 5 is arranged on one end of the track frame 3. A powersource such as an engine is connected to the sprocket wheel 5, and thesprocket wheel 5, driven by the power source, rotates about its axis. Onan outer peripheral surface of the sprocket wheel 5, a plurality ofprojections 51 are arranged which project radially outward. Theprojections 51 mesh with the track 2. The rotation of the sprocket wheel5 is thus transmitted to the track 2. The track 2, driven by therotation of the sprocket wheel 5, rotates in a circumferentialdirection.

The idler tumbler 4 is attached to the other end (opposite to the endwhere the sprocket wheel 5 is arranged) of the track frame 3. Further,on the track frame 3, in the region sandwiched between the sprocketwheel 5 and the idler tumbler 4, the track rollers 10 and the carrierrollers 11 are attached respectively on the ground contact side and onthe side opposite to the ground contact side. The idler tumbler 4, thetrack rollers 10, and the carrier rollers 11 have their outer peripheralsurfaces coming into contact with the inner peripheral surface of thetrack 2. As a result, the track 2, driven by the rotation of thesprocket wheel 5, rotates in the circumferential direction while beingguided by the idler tumbler 4, the sprocket wheel 5, the track rollers10, and the carrier rollers 11.

Referring to FIG. 2, an outer link 7 and an inner link 8 adjacent toeach other are connected by means of a connecting pin 12 and a bushing13. Each inner link 8 has two through holes 15 formed to penetrate in adirection perpendicular to the plane of rotation of the track 2. One andthe other of these two through holes 15 are formed on one end and theother end, respectively, in the longitudinal direction of the link. Eachouter link 7 has two through holes 15 formed to penetrate in a directionperpendicular to the plane of rotation of the track 2. One and the otherof these two through holes 15 are formed on one end and the other end,respectively, in the longitudinal direction of the link.

Referring to FIG. 3, the bushing 13 has a hollow cylindrical shape. Thebushing 13 has a smaller-diameter portion 139, having a smaller outerdiameter, formed on each end. The bushing 13 has an inner peripheralsurface 133 having a constant diameter over the length. The bushing 13is reduced in thickness in the smaller-diameter portions 139.

Referring to FIGS. 2 and 3, a pair of outer links 7 are arranged suchthat their two through holes 15 are aligned respectively as seen fromthe direction perpendicular to the plane of rotation of the track 2. Apair of inner links 8 are arranged such that their two through holes 15are aligned respectively as seen from the direction perpendicular to theplane of rotation of the track 2. An outer link 7 and an inner link 8adjacent to each other are arranged such that one through hole 15 of theouter link 7 and one through hole 15 of the inner link 8 are aligned asseen from the direction perpendicular to the plane of rotation of thetrack 2. A bushing 13 is arranged such that it is sandwiched between apair of inner links 8 and such that the smaller-diameter portion 139 oneach end is inserted into a through hole 15 of the corresponding innerlink 8. A connecting pin 12 is arranged to penetrate through the throughholes 15 of the neighboring outer and inner links 7 and 8, the holesbeing aligned as seen from the direction perpendicular to the plane ofrotation of the track 2, and through the space surrounded by the innerperipheral surface 133 of the corresponding bushing 13. The connectingpin 12 is arranged to penetrate through the bushing 13 in thelongitudinal direction.

The sprocket wheel 5 rotates in a circumferential direction, with itsouter peripheral surface 53 meshing with outer peripheral surfaces 131of the bushings 13 constituting the track 2. The outer peripheralsurface 53 of the sprocket wheel 5 and the outer peripheral surface 131of each bushing 13 are thus required to have high wear resistance. Thesprocket wheel 5 is a machine component that slides relative to anothercomponent, or, the bushing 13, while being in contact with the bushing13 in the contact region, or, the outer peripheral surface 53. Thebushing 13 is a machine component that slides relative to anothercomponent, or, the sprocket wheel 5, while being in contact with thesprocket wheel 5 in the contact region, or, the outer peripheral surface131.

Referring to FIGS. 4 and 5, the sprocket wheel 5 includes a base 50,made of a metal (steel), and an overlay 52 which covers the base 50 soas to form the outer peripheral surface 53, which is the contact region.The outer peripheral surface 53 which is the surface of the overlay 52has been smoothed. Here, the state in which the outer peripheral surface53, i.e. the surface of the overlay 52, is smoothed refers to the statein which surface profile affected by surface tension and the like at thetime of formation of the overlay 52 in the liquid state has beeneliminated from the surface of the overlay 52. In the presentembodiment, the outer peripheral surface 53, which is the surface of theoverlay 52, is a forged surface. The outer peripheral surface 53, or,the surface of the overlay 52 affected by surface tension and the likeduring formation of the overlay 52 in the liquid state, has beensmoothed by forging. For the metal forming the base 50, for example,carbon steel for machine structural use or alloy steel for machinestructural use specified in JIS standard (for example, S45C or SCM435,as well as manganese steel (SMn), chromium steel (SCr), orchromium-molybdenum steel (SCM) containing an equivalent amount ofcarbon) can be adopted.

A bushing 13 includes a base 134, and an overlay 132 which covers thebase 134 so as to form the outer peripheral surface 131, which is thecontact region. The outer peripheral surface 131 which is the surface ofthe overlay 132 has been smoothed. In the present embodiment, the outerperipheral surface 131, which is the surface of the overlay 132, is aforged surface. The outer peripheral surface 131, or, the surface of theoverlay 132 affected by surface tension and the like during formation ofthe overlay 52 in the liquid state, has been smoothed by forging. Forthe metal forming the base 134, for example, carbon steel for machinestructural use or alloy steel for machine structural use specified inJIS standard (for example, S45C or SCM435, as well as manganese steel(SMn), chromium steel (SCr), or chromium-molybdenum steel (SCM)containing an equivalent amount of carbon) can be adopted.

In the sprocket wheel 5 and the bushings 13 which are the machinecomponents in the present embodiment, the surfaces of the overlays 52and 132, which become the contact regions, have been smoothed. Thisprevents a local increase in contact pressure and other phenomena,making the components less damaging to the other components (bushings 13and sprocket wheel 5).

A description will now be made about the structure of an overlay formedon the sprocket wheel 5 and the bushings 13. FIG. 6 is a schematiccross-sectional view showing the structure of an overlay at and near itssurface. FIG. 7 is a schematic cross-sectional view showing thestructure at and around an interface between the overlay and a base.Referring to FIGS. 6 and 7, an overlay 90 (overlay 52 and overlay 132)formed on the sprocket wheel 5 and the bushings 13 includes a matrix 95made of a second metal, and hard particles 91 dispersed in the matrix95. The second metal forming the matrix 95 can be, for example, amixture of a metal derived from a welding wire and the metal (firstmetal) forming a base 100 (base 50 and base 134). As the hard particles91, particles having higher hardness than the matrix 95, for exampleparticles of cemented carbide, can be adopted. The overlay 90 has higherwear resistance than the base 100.

Referring to FIG. 6, the surface 90A of the overlay 90 is a forgedsurface. The hard particles 91 located in an overlay surface region 90B,which is a region within an average particle diameter of the hardparticles 91 from the surface 90A of the overlay 90, are arranged sideby side while being embedded in the overlay 90. This prevents the hardparticles 91 from being arranged protruding noticeably from the surface90A of the overlay 90. This consequently prevents the hard particles 91from falling off during the use of the sprocket wheel 5 and the bushings13, leading to improved wear resistance of the sprocket wheel 5 and thebushings 13.

The hard particles 91 located in the overlay surface region 90B may bearranged in contact with the surface 90A of the overlay 90, as shown inFIG. 6. With this, the region of a hard particle 91 exposed from thesurface 90A of the overlay 90 becomes small, which prevents the hardparticle 91 from falling off.

As shown in FIG. 6, among the hard particles 91 located in the overlaysurface region 90B, any hard particle 91 having a region exposed fromthe surface 90A of the overlay 90 may have an acute central angle θ (ofless than 90°) corresponding to that exposed region. With this, theregion of a hard particle 91 exposed from the surface 90A of the overlay90 becomes small, which prevents the hard particle 91 from falling off.

Referring to FIG. 7, in a region including an interface between theoverlay 90 and the base 100, the overlay 90 includes protrusions 99 thatprotrude toward the base 100. The protrusions 99 provide an anchoreffect to prevent the overlay 90 from coming off the base 100. Aprotrusion 99 receives at least a part of a hard particle 91. This morereliably prevents the overlay 90 from coming off the base 100. Thereexists the matrix 95 of the overlay 90 between the base 100 and the hardparticle 91 received in the protrusion 99. The hard particle 91 receivedin the protrusion 99 is not in contact with the base 100. The hardparticle 91 has its center located outside the protrusion 99 (i.e., apart of the hard particle 91 having a volume less than a half thereof isreceived in the protrusion 99). One hard particle 91 is received in oneprotrusion 99. Each protrusion 99 has a depth that is smaller than theradius of the hard particle 91 received in the protrusion 99.

A method for producing a sprocket wheel 5, which is the machinecomponent in the present embodiment, will now be described withreference to FIGS. 8 to 13. FIG. 8 is a flowchart schematicallyillustrating a method for producing a sprocket wheel which is themachine component. FIGS. 9, 10, 12, and 13 are schematic perspectiveviews illustrating the method for producing the sprocket wheel. FIG. 11is a schematic cross-sectional view illustrating a method for forming anoverlay.

Referring to FIG. 8, in the method for producing a sprocket wheel 5 inthe present embodiment, first, a base member preparing step is carriedout as a step S10. In this step S10, referring to FIG. 9, a base member61, which is to be a base 50 of the sprocket wheel 5, is prepared. Thebase member 61 is made of a metal forming the base 50. The base member61 is of a cylindrical shape. The base member 61 includes a pair of endfaces 61B and a side face 61A connecting the end faces 61B.

Next, an overlay forming step is carried out as a step S20. In this stepS20, referring to FIGS. 9 and 10, an overlay 63 is formed to cover apart of the side face 61A of the base member 61 prepared in the stepS10. The overlay 63 is formed over the entire area in the longitudinaldirection of the base member 61. The overlay 63 is formed in a part inthe circumferential direction (over approximately a half in thecircumferential direction) of the base member 61. The overlay 63 has astructure in which beads 62, extending in the longitudinal direction ofthe base member 61, are laid side by side in the circumferentialdirection, with no gaps therebetween.

The overlay 63 may be formed by, for example, overlaying welding usingCO₂ arc welding as described below. First, an overlay forming devicewill be described. Referring to FIG. 11, the overlay forming deviceincludes a welding torch 70 and a hard particles supplying nozzle 80.The welding torch 70 includes a welding nozzle 71 having a hollowcylindrical shape, and a contact tip 72 disposed inside the weldingnozzle 71 and connected to a power source (not shown). A welding wire73, while being in contact with the contact tip 72, is suppliedcontinuously to the tip end side of the welding nozzle 71. For thewelding wire, JIS YGW12, for example, can be adopted. A gap between thewelding nozzle 71 and the contact tip 72 is a flow path of shieldinggas. The shielding gas flowing through the flow path is discharged fromthe tip end of the welding nozzle 71. The hard particles supplyingnozzle 80 has a hollow cylindrical shape. Inside the hard particlessupplying nozzle 80, hard particles 91 are supplied, which aredischarged from the tip end of the hard particles supplying nozzle 80.

This overlay forming device can be used to form an overlay 63 throughthe following procedure. With a base member 61 as one electrode and thewelding wire 73 as another electrode, voltage is applied across the basemember 61 and the welding wire 73. This generates an arc 74 between thewelding wire 73 and the base member 61. The arc 74 is shielded from theambient air by the shielding gas discharged from the tip end of thewelding nozzle 71 along the arrows β. For the shielding gas, carbondioxide, for example, can be adopted. The heat in the arc 74 melts apart of the base member 61 and also melts the tip end of the weldingwire 73. The tip end of the welding wire 73 thus molten forms droplets,which transfer to the molten region of the base member 61. This forms amolten pool 92 which is a liquid region where the molten base member 61and the molten welding wire 73 are mixed together. The hard particles 91discharged from the hard particles supplying nozzle 80 are supplied tothis molten pool 92.

As the welding torch 70 and the hard particles supplying nozzle 80constituting the overlaying welding device move relatively in thedirection shown by the arrow α with respect to the base member 61, theposition where the molten pool 92 is formed move accordingly. The moltenpool 92 previously formed solidifies, resulting in a bead 62. The bead62 includes a matrix 95 formed by solidification of the molten pool 92,and hard particles 91 dispersed in the matrix 95. A plurality of suchbeads 62 are formed next to one another in the width direction, with nogaps therebetween, to cover a desired region on the side face 61A of thebase member 61, whereby formation of the overlay 63 is completed (seeFIG. 10). It should be noted that overlaying welding may be carried out,for example, under the following conditions: welding current of 230 A,welding voltage of 17 V, hard particles feed rate of 110 g/min, andexcess bead height of 4 mm. For the welding wire, JIS YGW11 may beadopted. For the hard particles, WC- or W₂C-based particles may beadopted.

Next, a hot forging step is carried out as a step S30. In this step S30,the base member 61 with the overlay 63 formed in the step S20 is hotforged. Referring to FIGS. 10 and 12, the base member 61 with theoverlay 63 formed is heated to a temperature enabling hot forging, andthen placed in a die having a cavity corresponding to a desired shape ofthe sprocket wheel 5, for forging. In the present embodiment, aplurality of arc-shaped parts, constituting an annular sprocket wheel 5,are produced by hot forging. The resultant parts are assembled in alater step, to obtain the annular sprocket wheel 5. With hot forging,the overlay 63 formed in the step S20 is worked. The base member 61 ishot forged such that the overlay 63 covers the outer peripheral surfaceof the sprocket wheel 5. In this manner, an overlay 52 having a smoothsurface, from which surface profile affected by surface tension in theliquid state and the like has been eliminated, is obtained. As a resultof hot forging, burr 59 is formed, as shown in FIG. 12. Thereafter,referring to FIGS. 12 and 13, die-cutting is carried out to remove theburr 59, whereby a part constituting the sprocket wheel 5 is obtained(see FIG. 13).

Referring to FIGS. 11 and 6, as the base member 61 having the overlay 63formed is hot forged, hard particles 91 that were protruding from asurface of the overlay 63 (beads 62) during formation of the overlay 63are pressed into the overlay 63 (beads 62). As a result, in the sprocketwheel 5, the hard particles 91 located in the overlay surface region 90Bare arranged side by side while being embedded in the overlay 90. Thehard particles 91 located in the overlay surface region 90B are arrangedin contact with the surface 90A of the overlay 90. Among the hardparticles 91 located in the overlay surface region 90B, any hardparticle 91 having a region exposed from the surface 90A of the overlay90 has an acute central angle θ (of less than 90°) corresponding to theexposed region. This prevents the hard particle 91 from falling offduring the use of the sprocket wheel 5, leading to improved wearresistance of the sprocket wheel 5.

Referring to FIGS. 11 and 7, as the base member 61 having the overlay 63formed is hot forged, protrusions 99 are formed in the overlay 90 inconsequence of the hard particles 91 that were located in the vicinityof the interface between the overlay 63 (beads 62) and the base member61 at the time of formation of the overlay 63 (beads 62). In aprotrusion 99, at least a part of a corresponding hard particle 91 isreceived. The above process simultaneously forms the surface region ofthe overlay 90 which is excellent in wear resistance with the hardparticles 91 arranged in contact with the surface 90A, and theprotrusions 99 which prevent the overlay 90 from coming off the base100.

Referring to FIG. 8, next, a heat treatment step is carried out as astep S40. In this step S40, the sprocket wheel 5 (the part constitutingthe sprocket wheel 5) obtained through hot forging in the step S30 issubjected to heat treatment. The heat treatment carried out in the stepS40 is, for example, quenching and tempering. This imparts desiredhardness and toughness to the base 50 of the sprocket wheel 5.Thereafter, in order to make it possible to mount the sprocket wheel 5on a support body (not shown), a region where no overlay 90 has beenformed is subjected to machining for the purposes of improvingdimensional accuracy, forming a mounting hole, and so on. The sprocketwheel 5 (the part constituting the sprocket wheel 5) in the presentembodiment is thus completed.

A method for producing a bushing 13, which is the machine component inthe present embodiment, will now be described with reference to FIGS. 8,14, and 15. FIG. 8 is a flowchart schematically illustrating a methodfor producing a bushing. FIGS. 14 and 15 are schematic perspective viewsillustrating the method for producing the bushing. The bushing 13 in thepresent embodiment can be produced through a similar procedure as thesprocket wheel 5 described above.

Referring to FIG. 8, in the method for producing a bushing 13 in thepresent embodiment, first, a base member preparing step is carried outas a step S10. In this step S10, referring to FIG. 14, a base member 64,which is to be a base 134 of the bushing 13, is prepared. The basemember 64 is made of a metal forming the base 134. The base member 64 isof a cylindrical shape. The base member 64 includes a pair of end faces64B and an outer peripheral surface 64A connecting the end faces 61B.

Next, an overlay forming step is carried out as a step S20. In this stepS20, referring to FIGS. 14 and 15, an overlay 63 is formed to cover apart of the outer peripheral surface 64A of the base member 61 preparedin the step S10. The overlay 63 is formed in the central portion in thelongitudinal direction of the base member 64. No overlay 63 is formed inboth end portions in the longitudinal direction of the base member 64.The overlay 63 is formed over the entire area in the circumferentialdirection of the base member 64. The overlay 63 has a structure in whichbeads 62, extending in the longitudinal direction of the base member 64,are laid side by side in the circumferential direction, with no gapstherebetween. The overlay 63 can be formed by, for example, overlayingwelding using CO₂ arc welding, as in the case of producing the sprocketwheel 5 described above. It should be noted that the overlay 63 may beformed in a part in the circumferential direction of the base member 64,for example over a half of the circumference, corresponding to theregion that is to come into contact with the sprocket wheel 5.

Next, a hot forging step is carried out as a step S30. In this step S30,the base member 64 with the overlay 63 formed in the step S20 is hotforged. Referring to FIG. 15 and FIGS. 3 to 5, the base member 64 withthe overlay 63 formed is heated to a temperature enabling hot forging,and then placed in a die having a cavity corresponding to a desiredshape of the bushing 13, for forging. With hot forging, the overlay 63formed in the step S20 is worked. The base member 64 is hot forged suchthat the overlay 63 covers the outer peripheral surface 131 of thebushing 13. In this manner, an overlay 132 having a smooth surface, fromwhich surface profile affected by surface tension in the liquid stateand the like has been eliminated, is obtained. The both ends in thelongitudinal direction of the base member 64, where no overlay 63 hasbeen formed, become the smaller-diameter portions 139 of the bushing 13.Thereafter, a pin hole is formed to which a connecting pin 12 is to beinserted for connection of the links 7 and 8 (see FIG. 2). Referring toFIG. 3, the pin hole is defined by the inner peripheral surface 133 andextends in the axial direction.

As the base member 64 having the overlay 63 formed is hot forged, hardparticles 91 that were protruding from a surface of the overlay 63(beads 62) during formation of the overlay 63 are pressed into theoverlay 63 (beads 62). As a result, in the bushing 13, the hardparticles 91 located in the overlay surface region 90B are arranged sideby side while being embedded in the overlay 90. The hard particles 91located in the overlay surface region 90B are arranged in contact withthe surface 90A of the overlay 90. Among the hard particles 91 locatedin the overlay surface region 90B, any hard particle 91 having a regionexposed from the surface 90A of the overlay 90 has an acute centralangle θ (of less than 90°) corresponding to the exposed region. Thisprevents the hard particle 91 from falling off during the use of thebushing 13, leading to improved wear resistance of the bushing 13.

As the base member 64 having the overlay 63 formed is hot forged, in thebushing 13, protrusions 99 are formed in the overlay 90 in consequenceof the hard particles 91 that were located in the vicinity of theinterface between the overlay 63 (beads 62) and the base member 64 atthe time of formation of the overlay 63 (beads 62). In a protrusion 99,at least a part of a corresponding hard particle 91 is received.

Referring to FIG. 8, next, a heat treatment step is carried out as astep S40. In this step S40, the bushing 13 obtained through hot forgingin the step S30 is subjected to heat treatment. The heat treatmentcarried out in the step S40 is, for example, quenching and tempering.This imparts desired hardness and toughness to the base 134 of thebushing 13. Thereafter, the smaller-diameter portions 139 of the bushing13 are subjected to machining for the purposes of improving dimensionalaccuracy, reducing surface roughness, and so on. The bushing 13 in thepresent embodiment is thus completed.

Second Embodiment

A machine component according to a second embodiment, which is anotherembodiment of the present invention, will now be described taking abucket tooth of a hydraulic excavator as an example. FIG. 16 is aschematic perspective view showing the structure of a bucket of ahydraulic excavator. FIG. 17 is a schematic plan view showing thestructure of a tooth. FIG. 18 is a schematic cross-sectional view takenalong the line XVIII-XVIII in FIG. 17.

Referring to FIG. 16, a bucket 201 in the present embodiment, which isattached to a tip end of an arm (not shown) of a hydraulic excavator, isfor excavating earth and sand. The bucket 201 includes: a main body 210,made up of a plate-like member and having an opening; a plurality of (inthe bucket 201 shown in FIG. 16, three) teeth 220 attached to the mainbody 210 to partially protrude from a periphery 212 of the opening ofthe main body 210 on its excavating side; and a mounting portion 230disposed on a side of the main body 210 opposite to the side where theteeth 220 are attached. The bucket 201 is supported by the arm of thehydraulic excavator via the mounting portion 230. When the bucket 201 isused for excavation, the teeth 220 enter into earth and sand first. Theteeth 220 are thus required to have high earth and sand abrasionresistance (wear resistance).

A tooth 220 includes a tip end 221 and a proximal end 222, as shown inFIG. 17. The tooth 220 is attached to the main body 210 at its proximalend 222 side, with its tip end 221 side protruding from the periphery212 of the opening of the bucket 201. The tooth 220 is used while beingin contact with another component which is the main body 210. The bucket201 enters into earth and sand from the tip end 221 side of the tooth220. The tip end 221 side of the tooth 220 thus requires particularlyhigh earth and sand abrasion resistance.

Referring to FIG. 18, a tooth 220 includes a base 225 made of a firstmetal, and an overlay 227 disposed in contact with the base 225 to covera covered region 225A that is a part of a surface of the base 225. Asthe first metal for the base 225, for example, carbon steel for machinestructural use or alloy steel for machine structural use specified inJIS standard (for example, S45C or SCM435, as well as manganese steel(SMn), chromium steel (SCr), or chromium-molybdenum steel (SCM)containing an equivalent amount of carbon) can be adopted. In an overlayedge portion 229 corresponding to a boundary between the covered region225A and an exposed region 225B that is a region other than the coveredregion 225A on the surface of the base 225, the exposed region 225B anda surface 227A of the overlay 227 are flush with each other to form aforged surface. The surface 227A of the overlay 227 is entirely theforged surface.

FIG. 19 is a schematic cross-sectional view showing the structure of atooth with an overlay, as a Comparative example. In the case of formingan overlay for improving wear resistance at and near a tip end of atooth, it is common to form the overlay on a steel base having a desiredshape. Referring to FIG. 19, the tooth 920 of the Comparative example,which is a typical tooth having an overlay, includes a tip end 921 and aproximal end 922. An overlay 927 is formed on the tip end 921 side ofthe tooth 920. The overlay 927 is formed by overlaying welding, forexample, to cover a covered region 925A of a base 925 that has beenshaped into a desired shape. Thus, in overlay edge portions 929A, 929Bcorresponding to boundaries between the covered region 925A and exposedregions 925B, 925C other than the covered region 925A, steps are formedbetween the exposed regions 925B, 925C and a surface 927A of the overlay927. Such steps increase penetration resistance of the tooth 920 inearth and sand. Further, the overlay 927 is formed after the base 925 isshaped. It is difficult to form the overlay 927 in the vicinity of thetip end 921. Thus, in the region including the tip end 921, the tip-endexposed region 925C is formed which is uncovered with the overlay 927.This tip-end exposed region 925C having low wear resistance acceleratesthe progress of wear and increases the replacement frequency of thetooth 920.

Referring to FIG. 18, according to the tooth 220 in the presentembodiment, the exposed region 225B and the surface 227A of the overlay227 are flush with each other in the overlay edge portion 229. This canprevent an increase in penetration resistance otherwise caused by a stepin the overlay edge portion 229. With the overlay edge portion 229included in the forged surface, the working step such as cutting formaking the exposed region 225B and the surface 227A of the overlay 227flush with each other can be omitted. Consequently, the working on theoverlay edge portion 229 where the difference in hardness is great andthe working on the overlay 227 which is high in hardness can be avoided.Thus, according to the tooth 220 in the present embodiment, it ispossible to prevent a disadvantage that would otherwise be caused byforming the overlay 227. Further, an overlay may be formed on a basemember and then forging may be performed to shape a region including thetip end 221. By doing so, the region including the tip end 221 canreadily be covered with the overlay 227, as shown in FIG. 18, and atooth 220 having high wear resistance can be obtained.

Referring to FIGS. 6 and 7, the overlay 90 (overlay 227) formed on thetooth 220 includes, as in the case of the sprocket wheel 5 and thebushing 13 in the first embodiment described above, a matrix 95 made ofa second metal, and hard particles 91 dispersed in the matrix 95. Thesecond metal forming the matrix 95 can be, for example, a mixture of ametal derived from a welding wire and a metal (first metal) forming thebase 100 (base 225). As the hard particles 91, particles having higherhardness than the matrix 95, for example particles of cemented carbide,can be adopted. The overlay 90 has higher earth and sand abrasionresistance (wear resistance) than the base 100.

Referring to FIG. 6, the surface 90A of the overlay 90 is a forgedsurface. The hard particles 91 located in an overlay surface region 90B,which is a region within an average particle diameter of the hardparticles 91 from the surface 90A of the overlay 90, are arranged sideby side while being embedded in the overlay 90. This prevents the hardparticles 91 from being arranged protruding noticeably from the surface90A of the overlay 90. This consequently prevents the hard particles 91from falling off during the use of the tooth 220, leading to improvedwear resistance of the tooth 220.

The hard particles 91 located in the overlay surface region 90B may bearranged in contact with the surface 90A of the overlay 90, as shown inFIG. 6. With this, the region of a hard particle 91 exposed from thesurface 90A of the overlay 90 becomes small, which prevents the hardparticle 91 from falling off.

As shown in FIG. 6, among the hard particles 91 located in the overlaysurface region 90B, any hard particle 91 having a region exposed fromthe surface 90A of the overlay 90 may have an acute central angle θ (ofless than 90°) corresponding to that exposed region. With this, theregion of a hard particle 91 exposed from the surface 90A of the overlay90 becomes small, which prevents the hard particle 91 from falling off.

Referring to FIG. 7, the overlay 90 includes protrusions 99 thatprotrude toward the base 100 in a region including an interface betweenthe overlay 90 and the base 100. The protrusions 99 provide an anchoreffect to prevent the overlay 90 from coming off the base 100. Aprotrusion 99 receives at least a part of a hard particle 91. This morereliably prevents the overlay 90 from coming off the base 100. Thereexists the matrix 95 of the overlay 90 between the base 100 and the hardparticle 91 received in the protrusion 99. The hard particle 91 receivedin the protrusion 99 is not in contact with the base 100. The hardparticle 91 has its center located outside the protrusion 99 (i.e., apart of the hard particle 91 having a volume less than a half thereof isreceived in the protrusion 99). One hard particle 91 is received in oneprotrusion 99. Each protrusion 99 has a depth that is smaller than theradius of the hard particle 91 received in the protrusion 99.

A method for producing a tooth 220 will now be described. FIG. 8 is aflowchart schematically illustrating a method for producing a tooth,which is the machine component. FIGS. 20 and 21 are schematiccross-sectional views illustrating the method for producing the tooth.

Referring to FIG. 8, in the method for producing a tooth 220 in thepresent embodiment, first, a base member preparing step is carried outas a step S10. In this step S10, referring to FIG. 20, a base member250, which is to be a base 225 of the tooth 220, is prepared. The basemember 250 is made of a first metal. The base member 250 is of acylindrical shape. The base member 250 has a cylindrical shape includingone end face 252, another end face 253, and a side face 251 connectingthe one end face 252 and the other end face 253. A first chamferedportion 252A is formed in a region where the one end face 252 and theside face 251 are connected. A second chamfered portion 253A is formedin a region where the other end face 253 and the side face 251 areconnected. Referring to FIGS. 20 and 18, the one end face 252 side ofthe base member 250 corresponds to the tip end 221 side of the tooth220, and the other end face 253 side of the base member 250 correspondsto the proximal end 222 side of the tooth 220.

Next, an overlay forming step is carried out as a step S20. In this stepS20, referring to FIGS. 20 and 21, an overlay 260 is formed in contactwith a covered region 251A that is a part of a surface of the basemember 250 prepared in the step S10, to cover the covered region 251A.The overlay 260 is formed such that it will cover a desired region ofthe base 225 when hot forging is carried out, which will be describedlater. The covered region 251A can be determined in advance through asimulation of hot forging using a finite element method, for example. Inthe present embodiment, referring to FIG. 21, the overlay 260 is formedto cover the one end face 252 side of the side face 251, the firstchamfered portion 252A, and the one end face 252. The overlay 260 can beformed by overlaying welding using CO₂ arc welding, as in the case ofthe first embodiment described above.

Next, a hot forging step is carried out as a step S30. In this step S30,the base member 250 with the overlay 260 formed in the step S20 is hotforged. Referring to FIGS. 21 and 18, the base member 250 with theoverlay 260 formed is heated to a temperature enabling hot forging, andthen placed in a die having a cavity corresponding to a desired shape ofthe tooth 220, for forging. With this hot forging, a region of the basemember 250 including an overlay edge portion 259 is worked. As a resultof hot forging, the overlay edge portion 259 becomes the overlay edgeportion 229 of the tooth 220. With the overlay edge portion 259 workedin hot forging, the tooth 220 is obtained which has the exposed region225B and the surface 227A of the overlay 227 flush with each other inthe overlay edge portion 229. In the overlay edge portion 229, theexposed region 225B and the surface 227A of the overlay 227 form aflush, forged surface corresponding to the region of the surface of thedie used in the hot forging where the overlay edge portion 259 isworked. In the overlay edge portion 229, the exposed region 225B and thesurface 227A of the overlay 227 form a flush surface corresponding tothe shape of the die for forging. The overlay edge portion 229 isincluded in the forged surface.

As the base member 250 having the overlay 260 formed is hot forged, hardparticles 91 that were protruding from a surface of the overlay 260during formation of the overlay 260 are pressed into the overlay 260. Asa result, in the tooth 220, the hard particles 91 located in the overlaysurface region 90B are arranged side by side while being embedded in theoverlay 90. The hard particles 91 located in the overlay surface region90B are arranged in contact with the surface 90A of the overlay 90.Among the hard particles 91 located in the overlay surface region 90B,any hard particle 91 having a region exposed from the surface 90A of theoverlay 90 has an acute central angle θ (of less than 90°) correspondingto the exposed region (see FIG. 6). This prevents the hard particle 91from falling off during the use of the tooth 220, leading to improvedwear resistance of the tooth 220.

As the base member 250 having the overlay 260 formed is hot forged, inthe tooth 220, protrusions 99 are formed in the overlay 90 inconsequence of the hard particles 91 that were located in the vicinityof the interface between the overlay 260 and the base member 250 at thetime of formation of the overlay 260. In a protrusion 99, at least apart of a corresponding hard particle 91 is received (see FIG. 7).

Referring to FIG. 8, next, a heat treatment step is carried out as astep S40. In this step S40, the tooth 220 obtained through hot forgingin the step S30 is subjected to heat treatment. The heat treatmentcarried out in the step S40 is, for example, quenching and tempering.This imparts desired hardness and toughness to the base 225 of the tooth220. Through the above procedure, the tooth 220 in the presentembodiment is completed.

Third Embodiment

A machine component according to a third embodiment, which is anotherembodiment of the present invention, will now be described taking atooth of a crusher as an example. FIG. 22 is a schematic perspectiveview showing the structure of a crusher. FIG. 23 is a schematicperspective view showing the structure of a stationary-section externaltooth of the crusher. FIG. 24 is a schematic cross-sectional view takenalong the line XXIV-XXIV in FIG. 23. FIG. 25 is a schematiccross-sectional view taken along the line XXV-XXV in FIG. 24.

Referring to FIG. 22, a crusher 301 in the present embodiment, which isattached to a tip end of an arm of a work machine, is for crushingconcrete or other materials to be crushed. The crusher 301 includes amain frame 310, a stationary section 320 fixed to the main frame 310,and a movable section 330 turnably attached to the main frame 310 suchthat it can be opened and closed with respect to the stationary section320. The movable section 330 is connected to a hydraulic cylinder (notshown) disposed in the main frame 310. The movable section 330, drivenby the hydraulic cylinder, turns to be opened and closed with respect tothe stationary section 320.

The stationary section 320 has a tip end on which a plurality of (in thepresent embodiment, three) stationary-section external teeth 321 areattached spaced apart from each other. In the stationary section 320, acrusher plate 322 is placed on the region that opposes the movablesection 330 when the movable section 330 is closed with respect to thestationary section 320. The crusher plate 322 has a plurality of throughholes formed, and recesses 323 are formed in the regions of thestationary section 320 exposed from the respective through holes. Themovable section 330 has a tip end on which a plurality of (in thepresent embodiment, two) movable-section external teeth 331 are attachedspaced apart from each other. In the movable section 330, a plurality ofmovable-section internal teeth 333 are disposed in the region thatopposes the stationary section 320 when the movable section 330 isclosed with respect to the stationary section 320. When the movablesection 330 is closed with respect to the stationary section 320, thetwo movable-section external teeth 331 respectively enter the two spacesformed by the three stationary-section external teeth 321. When themovable section 330 is closed with respect to the stationary section320, the movable-section internal teeth 333 enter the correspondingrecesses 323 in the stationary section 320. With the crusher 301 havingsuch a structure, concrete or other materials to be crushed are fedbetween the movable section 330 and the stationary section 320 in thestate where the movable section 330 is open with respect to thestationary section 320, and the movable section 330 is then closed withrespect to the stationary section 320, whereby the materials arecrushed. When the materials are being crushed, the stationary-sectionexternal teeth 321, the movable-section external teeth 331, and themovable-section internal teeth 333 directly come into contact with theconcrete or other materials to be crushed. The stationary-sectionexternal teeth 321, the movable-section external teeth 331, and themovable-section internal teeth 333, which are the teeth of the crusher,are thus required to have high wear resistance. For improving the wearresistance, overlays can be formed on the teeth of the crusher, or, thestationary-section external teeth 321, the movable-section externalteeth 331, and the movable-section internal teeth 333. Hereinafter, thestructure of a stationary-section external tooth 321, as an example ofthe tooth of the crusher, will be described.

Referring to FIG. 23, a stationary-section external tooth 321 has: a tipend face 321A which is a flat surface on the tip end side; a proximalend face 321D which is a flat surface on the proximal end side; a firstside face 321B which is a flat surface connecting the tip end face 321Aand the proximal end face 321D and supposed to face the movable section330 side; a second side face 321E which is a curved surface connectingthe tip end face 321A and the proximal end face 321D and locatedopposite to the first side face 321B; and two third side faces 321C eachof which is a flat surface connecting the tip end face 321A and theproximal end face 321D and also connecting the first side face 321B andthe second side face 321E. The stationary-section external tooth 321 isattached to the stationary section 320 at the proximal end face 321D.

Referring to FIGS. 24 and 25, the stationary-section external tooth 321includes a base 325 made of a first metal, and an overlay 327 disposedin contact with the base 325 so as to cover a part of the surface of thebase 325. The overlay 327 covers the entire area of the tip end face321A, and a part of each of the first side face 321B, the second sideface 321E, and the third side faces 321C. The first side face 321B andthe second side face 321E have greater proportions of their surfacescovered by the overlay 327 as compared to the third side faces 321C. Nooverlay 327 is formed on the proximal end face 321D. For the first metalforming the base 325, for example, carbon steel for machine structuraluse or alloy steel for machine structural use specified in JIS standard(for example, S45C or SCM435, as well as manganese steel (SMn), chromiumsteel (SCr), or chromium-molybdenum steel (SCM) containing an equivalentamount of carbon) can be adopted.

Referring to FIGS. 6 and 7, the overlay 90 (overlay 327) formed on thestationary-section external tooth 321 includes a matrix 95 made of asecond metal, and hard particles 91 dispersed in the matrix 95, as inthe case of the sprocket wheel 5 and the bushings 13 in the firstembodiment described above. The second metal forming the matrix 95 canbe, for example, a mixture of a metal derived from a welding wire andthe metal (first metal) forming the base 100 (base 325). As the hardparticles 91, particles having higher hardness than the matrix 95, forexample particles of cemented carbide, can be adopted. The overlay 90has higher wear resistance than the base 100.

Referring to FIG. 6, the surface 90A of the overlay 90 is a forgedsurface. The hard particles 91 located in an overlay surface region 90B,which is a region within an average particle diameter of the hardparticles 91 from the surface 90A of the overlay 90, are arranged sideby side while being embedded in the overlay 90. This prevents the hardparticles 91 from being arranged protruding noticeably from the surface90A of the overlay 90. This consequently prevents the hard particles 91from falling off during the use of the stationary-section external tooth321, leading to improved wear resistance of the stationary-sectionexternal tooth 321.

The hard particles 91 located in the overlay surface region 90B may bearranged in contact with the surface 90A of the overlay 90, as shown inFIG. 6. With this, the region of a hard particle 91 exposed from thesurface 90A of the overlay 90 becomes small, which prevents the hardparticle 91 from falling off.

As shown in FIG. 6, among the hard particles 91 located in the overlaysurface region 90B, any hard particle 91 having a region exposed fromthe surface 90A of the overlay 90 may have an acute central angle θ (ofless than 90°) corresponding to that exposed region. With this, theregion of a hard particle 91 exposed from the surface 90A of the overlay90 becomes small, which prevents the hard particle 91 from falling off.

Referring to FIG. 7, in a region including an interface between theoverlay 90 and the base 100, the overlay 90 includes protrusions 99 thatprotrude toward the base 100. The protrusions 99 provide an anchoreffect to prevent the overlay 90 from coming off the base 100. Aprotrusion 99 receives at least a part of a hard particle 91. This morereliably prevents the overlay 90 from coming off the base 100. Thereexists the matrix 95 of the overlay 90 between the base 100 and the hardparticle 91 received in the protrusion 99. The hard particle 91 receivedin the protrusion 99 is not in contact with the base 100. The hardparticle 91 has its center located outside the protrusion 99 (i.e., apart of the hard particle 91 having a volume less than a half thereof isreceived in the protrusion 99). One hard particle 91 is received in oneprotrusion 99. Each protrusion 99 has a depth that is smaller than theradius of the hard particle 91 received in the protrusion 99.

A method for producing a stationary-section external tooth 321 will nowbe described. FIG. 8 is a flowchart schematically illustrating a methodfor producing a stationary-section external tooth, which is the machinecomponent. FIGS. 26 and 27 are schematic perspective views illustratinga method for producing a stationary-section external tooth of a crusher.FIGS. 28 and 29 are schematic cross-sectional views illustrating themethod for producing the stationary-section external tooth of thecrusher. FIG. 28 is a schematic cross-sectional view taken along theline XXVIII-XXVIII in FIG. 27. FIG. 29 is a schematic cross-sectionalview taken along the line XXIX-XXIX in FIG. 28. FIGS. 30 and 31 areschematic perspective views illustrating another method for producingthe stationary-section external tooth of the crusher. FIGS. 32 and 33are schematic cross-sectional views illustrating the other method forproducing the stationary-section external tooth of the crusher. FIG. 32is a schematic cross-sectional view taken along the line XXXII-XXXII inFIG. 31. FIG. 33 is a schematic cross-sectional view taken along theline XXXIII-XXXIII in FIG. 32.

Referring to FIG. 8, in the method for producing a stationary-sectionexternal tooth 321 in the present embodiment, first, a base memberpreparing step is carried out as a step S10. In this step S10, referringto FIG. 26, a base member 350, which is to be a base 325 of thestationary-section external tooth 321, is prepared. The base member 350is made of a first metal. The base member 350 has a cylindrical shapeincluding a first end face 350A, a second end face 350D, and a side face350F.

Next, an overlay forming step is carried out as a step S20. In this stepS20, referring to FIGS. 27 to 29, an overlay 360 is formed to cover thefirst end face 350A and a region on the first end face 350A side of theside face 350F of the base member 350 prepared in the step S10. Theoverlay 360 can be formed with a plurality of beads 362, as in the caseof the first embodiment. The first end face 350A is entirely covered bythe overlay 360. No overlay 360 is formed on the second end face 350D.On the side face 350F, a region within a predetermined distance in theaxial direction from the region connected to the first end face 350A iscovered with the overlay 360. While the overlay 360 on the side face350F basically has a uniform length in the axial direction, it has ashorter length in a pair of regions symmetrical with respect to thecenter axis. Corresponding to these regions, on the side face 350F,projecting exposed regions 350G are formed which are uncovered with anoverlay and projecting toward the first end face 350A. The overlay 360can be formed by overlaying welding using CO₂ arc welding, as in thecase of the first embodiment described above.

Next, a hot forging step is carried out as a step S30. In this step S30,the base member 350 with the overlay 360 formed in the step S20 is hotforged. Referring to FIGS. 27 to 29 and FIGS. 24 and 25, the base member350 with the overlay 360 formed is heated to a temperature enabling hotforging, and then placed in a die having a cavity corresponding to adesired shape of the stationary-section external tooth 321, for forging.With hot forging, the overlay 360 formed in the step S20 is worked. As aresult of hot forging, the first end face 350A and the second end face350D become regions corresponding respectively to the tip end face 321Aand the proximal end face 321D of the stationary-section external tooth321. Of the side face 350F, the regions in the circumferential directionwhere the projecting exposed regions 350G have been formed become thethird side faces 321C of the stationary-section external tooth 321, andthe regions other than the regions where the projecting exposed regions350G have been formed become the first side face 321B and the secondside face 321E of the stationary-section external tooth 321.

As the base member 350 having the overlay 360 formed is hot forged, hardparticles 91 that were protruding from a surface of the overlay 360during formation of the overlay 360 are pressed into the overlay 360. Asa result, in the stationary-section external tooth 321, the hardparticles 91 located in the overlay surface region 90B are arranged sideby side while being embedded in the overlay 90. The hard particles 91located in the overlay surface region 90B are arranged in contact withthe surface 90A of the overlay 90 (see FIG. 6). Among the hard particles91 located in the overlay surface region 90B, any hard particle 91having a region exposed from the surface 90A of the overlay 90 has anacute central angle θ (of less than 90°) corresponding to the exposedregion. This prevents the hard particle 91 from falling off during theuse of the stationary-section external tooth 321, leading to improvedwear resistance of the stationary-section external tooth 321.

As the base member 350 having the overlay 360 formed is hot forged, inthe stationary-section external tooth 321, protrusions 99 are formed inthe overlay 90 in consequence of the hard particles 91 that were locatedin the vicinity of the interface between the overlay 360 and the basemember 350 at the time of formation of the overlay 360. In a protrusion99, at least a part of a corresponding hard particle 91 is received (seeFIG. 7).

Referring to FIG. 8, next, a heat treatment step is carried out as astep S40. In this step S40, the stationary-section external tooth 321obtained through hot forging in the step S30 is subjected to heattreatment. The heat treatment carried out in the step S40 is, forexample, quenching and tempering. This imparts desired hardness andtoughness to the base 325 of the stationary-section external tooth 321.Thereafter, the regions where no overlay 360 has been formed aresubjected to machining for the purposes of improving dimensionalaccuracy, and so on. The stationary-section external tooth 321 in thepresent embodiment is thus completed.

It should be noted that in the above-described steps, from thestandpoint of facilitating hot forging after formation of the overlay360, preforming may be carried out prior to the formation of the overlay360. Specifically, referring to FIGS. 26 and 30, a base member 350having a cylindrical shape is prepared in the step S10, and then, thefirst end face 350A side is subjected to preforming. With thispreforming, the first end face 350A is formed into a rectangular shape,and first chamfered portions 350B and second chamfered portions 350C areformed connected respectively to the long sides and the short sides ofthe first end face 350A.

Next, in the step S20, referring to FIGS. 31 to 33, an overlay 360 isformed to cover the first end face 350A, the first chamfered portions350B, and the second chamfered portions 350C of the pre-formed basemember 350. The first end face 350A is entirely covered with the overlay360. No overlay 360 is formed on the second end face 350D. The overlay360 extending in the axial direction is longer in length in the regionsextending over the first chamfered portions 350B (see FIG. 33), than inthe regions extending over the second chamfered portions 350C (see FIG.32). Then, in the step S30, hot forging is carried out, whereby thefirst end face 350A and the second end face 350D become the regionscorresponding to the tip end face 321A and the proximal end face 321D,respectively, of the stationary-section external tooth 321. The regionswhere the overlay 360 extends short in the axial direction become thethird side faces 321C of the stationary-section external tooth 321, andthe regions where the overlay 360 extends long become the first sideface 321B and the second side face 321E of the stationary-sectionexternal tooth 321.

In each of the methods for producing the machine components in the firstthrough third embodiments described above, when forming the overlay onthe base member, a surface portion of the base member corresponding tothe region of the base member where the overlay is to be formed may beremoved in advance, or, an undercut portion may be formed in the basemember, before formation of the overlay. This reduces the deformationamount of the overlay at the time of forging, thereby preventing, forexample, wrinkling of the forged overlay.

Examples

A tooth 220 was produced through a similar procedure as the producingmethod described in the second embodiment, and the obtained tooth wassubjected to a test to examine the structure of the overlay and the like(Example). For comparison, a tooth was produced in a similar producingmethod, except that the overlay forming step (step S20) was omitted, andan overlay was formed by overlaying welding after the heat treatment.The obtained tooth was subjected to a similar test (Comparativeexample). The dies used for hot forging in the Example and in theComparative example were of the same shape.

FIG. 34 is a photograph showing a cross section of the tooth 220 of theExample. Referring to FIG. 34, in the overlay edge portion 229, theexposed region 225B and the surface 227A of the overlay 227 are flushwith each other to form a forged surface. It is thus confirmed that thetooth 220 in the second embodiment can be produced by the producingmethod in the second embodiment. No cracking is seen between the overlay227 and the base 225. No problem is found resulting from hot forgingperformed after the formation of the overlay.

FIG. 35 is an optical micrograph obtained by imaging a surface and itsvicinity of the overlay of the Example. FIG. 36 is an optical micrographobtained by imaging a surface and its vicinity of the overlay of theComparative example. As shown in FIG. 36, in the overlay of theComparative example, which has been formed by overlaying welding and notworked by forging thereafter, hard particles 91 protrude noticeably fromthe surface 90A of the overlay. Referring to FIG. 35, in the overlay ofthe Example, which has been formed and then worked by forging, hardparticles 91 located in the surface region are arranged side by side inthe state being embedded in the overlay (matrix 95). The hard particles91 are aligned in contact with the surface 90A of the overlay. A hardparticle 91 having a region exposed from the surface 90A of the overlay90 has an acute central angle θ (of less than 90°) corresponding to theexposed region. This is presumably because, during the process in whichthe overlay is worked by forging, the hard particles 91 that wereprotruding from the surface 90A of the overlay are pressed into thematrix 95 having relatively low hardness.

FIG. 37 is an optical micrograph obtained by imaging an interfacebetween the overlay and the base and its vicinity of the Example. FIG.38 is an optical micrograph obtained by imaging an interface between theoverlay and the base and its vicinity of the Comparative example. Asshown in FIG. 38, in the Comparative example where the overlay has beenformed by overlaying welding and not worked by forging thereafter, theinterface between the overlay (matrix 95) and the base 100 is flat.Referring to FIG. 37, in the Example which has been worked by forgingafter formation of the overlay, in the region including the interfacebetween the overlay (matrix 95) and the base 100, protrusions 99 areformed with the overlay (matrix 95) protruding toward the base 100. Ineach protrusion 99, a part of a corresponding hard particle 91 isreceived. It is considered that the protrusions 99 have been formedwhile the overlay was worked by forging, in consequence of the hardparticles 91 present in the vicinity of the interface with the basemember. A hard particle 91 that has contributed to the formation of aprotrusion 99 has at least a part received in the protrusion 99.

While the sprocket wheel and the bushings as the tracked undercarriagecomponents, the bucket teeth for a hydraulic excavator, and the teethfor a crusher have been described as examples of the machine componentsof the present invention in the above embodiments, the machinecomponents of the present invention are not limited thereto. The presentinvention is widely applicable to the machine components in which anoverlay having hard particles dispersed in a matrix is formed.

It should be understood that the embodiments and examples disclosedherein are illustrative and non-restrictive in every respect. The scopeof the present invention is defined by the terms of the claims, ratherthan the description above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

The machine component and its producing method according to the presentinvention are applicable particularly advantageously to a machinecomponent for which improved wear resistance is desired and to itsproducing method.

DESCRIPTION OF REFERENCE NUMERALS

1: track travel device; 2: track; 3: track frame; 4: idler tumbler; 5:sprocket wheel; 6: track shoe; 7: outer link; 8: inner link; 9: tracklink; 10: track roller; 11: carrier roller; 12: connecting pin; 13:bushing; 15: through hole; 50: base; 51: projection; 52: overlay; 53:outer peripheral surface; 59: burr; 61: base member; 61A: side face;61B: end face; 62: bead; 63: overlay; 64: base member; 64A: outerperipheral surface; 64B: end face; 70: welding torch; 71: weldingnozzle; 72: contact tip; 73: welding wire; 74: arc; 80: hard particlessupplying nozzle; 90: overlay; 90A: surface; 90B: overlay surfaceregion; 91: hard particle; 92: molten pool; 95: matrix; 99: protrusion;100: base; 131: outer peripheral surface; 132: overlay; 133: innerperipheral surface; 134: base; 139: smaller-diameter portion; 201:bucket; 210: main body; 212: periphery of opening; 220: tooth; 221: tipend; 222: proximal end; 225: base; 225A: covered region; 225B: exposedregion; 227: overlay; 227A: surface; 229: overlay edge portion; 230:mounting portion; 250: base member; 251: side face; 251A: coveredregion; 252: one end face; 252A: first chamfered portion; 253: other endface; 253A: second chamfered portion; 259: overlay edge portion; 260:overlay; 301: crusher; 310: main frame; 320: stationary section; 321:stationary-section external tooth; 321A: tip end face; 321B: first sideface; 321C: third side face; 321D: proximal end face; 321E: second sideface; 322: crusher plate; 323: recess; 325: base; 327: overlay; 330:movable section; 331: movable-section external tooth; 333:movable-section internal tooth; 350: base member; 350A: first end face;350B: first chamfered portion; 350C: second chamfered portion; 350D:second end face; 350F: side face; 350G: projecting exposed region; 360:overlay; and 362: bead.

The invention claimed is:
 1. A machine component, comprising: a base made of a first metal; and an overlay disposed in contact with the base to cover at least a part of a surface of the base, the overlay including a matrix made of a second metal, and hard particles dispersed in the matrix and having a higher hardness than the matrix, the overlay having a forged surface, and the hard particles located in an overlay surface region being arranged side by side and being embedded in the overlay, the overlay surface region being a region within an average particle diameter of the hard particles from the surface of the overlay, wherein: in an interface region that includes an interface between the overlay and the base, the overlay includes a protrusion that protrudes toward the base; the protrusion has at least a part of a hard particle of the hard particles received therein; and none of the hard particles have a center which is located inside of the protrusion.
 2. The machine component according to claim 1, wherein the hard particles located in the overlay surface region are arranged in contact with the surface of the overlay.
 3. The machine component according to claim 1, wherein among the hard particles located in the overlay surface region, any hard particle having a region exposed from the surface of the overlay has an acute central angle corresponding to the region exposed from the surface of the overlay.
 4. The machine component according to claim 1, wherein the machine component is a component of a tracked undercarriage, a bucket tooth, or a tooth of a crusher.
 5. A method for producing a machine component, comprising the steps of: preparing a base member made of a first metal; forming an overlay to cover at least a part of a surface of the base member, the overlay including a matrix made of a second metal and hard particles dispersed in the matrix, the hard particles having a higher hardness than the matrix; and forging the base member after the overlay is formed on the base member such that the overlay is worked, wherein: in an interface region that includes an interface between the overlay and the base, the overlay includes a protrusion that protrudes toward the base; the protrusion has at least a part of a hard particle of the hard particles received therein; and none of the hard particles have a center which is located inside of the protrusion.
 6. The machine component producing method according to claim 5, wherein the step of forging the base member includes hot forging the base member.
 7. The machine component producing method according to claim 5, wherein the machine component is a component of a tracked undercarriage, a bucket tooth, or a tooth of a crusher. 